It is clear and crisp, a perfect morning for hot-air ballooning. The sun rises as a balloon floats over a peaceful valley. Suddenly, the pilot opens the blast valve on his burner system to add heat to the balloon's envelope. Conversations are cut off in mid-sentence and passengers cringe as a loud roar comes from the burners. Far below wildlife scatters. Suddenly the valley is not the peaceful place it was a few seconds before.
Burner noise is one of the leading problems facing hot-air ballooning. Burner noise not only causes passenger discomfort and landowners relation problems, but it can also lead to pilot, crew, even passenger hearing loss.
By its very nature, you would expect a balloon burner to be noisy. The propane-powered hot air balloon burner is used to heat the air contained within a balloon envelope. The burner must produce enough heat to raise 66 cubic feet of air to a temperature of approximately 100· F above ambient temperature to produce just one pound of lift. Since most systems, including passengers, weigh around 1000 pounds, a hot-air balloon burner must be incredibly powerful (an output of 10 to 11 million BTUs) to produce the heat necessary to lift its weight.
The hot air balloon burner generates a level of noise between that of a freight train (88 decibels) and a circular saw (107 decibels). Noise at levels of 155 decibels has been known to burn the skin without heat, and sounds reaching 180 decibels can even kill. However, the decibel is a logarithmic rating system that accounts for large differences in audible sound intensities. So, modern hot air balloon burners, which generate a noise level of around 97 decibels, or older balloon burners, which operate at around 105 decibels, produce significantly less noise than those deadly levels of sound. The decibel scale accounts for the human perception of a doubling of loudness for every increase of 10 decibels, so a 70-decibel sound level is twice as loud as a 60-decibel sound level.
The burners produce noise in two ways. First, noise is generated when propane is ejected at a high rate of speed out of jets used to vaporize it. Second, there is noise produced from the combustion of oxygen and the vaporized propane. The noise of combustion is a design tradeoff for burner efficiency, says Mark West, a design engineer for balloon manufacturer Aerostar. "If the combustion takes place instantaneously and you have very thorough combustion, you actually produce the highest amount of noise because you are producing the greatest expansion of gas," he says.
Loud noises should be a concern to balloonists for a number of reasons. First, the body reacts to noise as a danger signal. When we are exposed to loud noise, blood pressure rises, heart rate and breathing speed up, muscles tense, etc. Loud sounds even affect the level of white blood cells, lowering the efficiency of the immune system. Second, and perhaps of greater concern, continued exposure to high levels of noise may lead to gradual hearing loss.
Loud noise can damage the inner ear, leading to permanent hearing loss. Hair cells in the inner ear, the cochlea, are like blades of grass growing in a lawn. In their normal healthy position, they are standing up vertically. But, where there is exposure to high noise levels without hearing protection, the force of noise pushes the hair cells down to a leaning position. With a temporary hearing loss, the hair cells eventually spring back to their normal position after a period of non-exposure. Once there has been repeated exposure without hearing protection, the hair cells do not recover and stay in a somewhat horizontal position.
A person experiencing a hearing loss will initially have difficulty discriminating between certain higher pitched consonant sounds. For example, if someone were to say "witch," it would sound more like "wish." The sounds "ch" and "sh" are both at high frequencies, and if hearing is not normal, it will be difficult to tell the two apart. With repeated exposure, hearing loss can become worse and will begin to affect more than just the high frequency range of hearing. "There is currently no way to restore life to dead hair cells and nerve endings in humans," explains Dr. Georgia Gates, a director of the Bloedell Hearing Research Center at the University of Washington. "Noise doesn't have to hurt your ears to hurt your hearing."
OSHA, the Occupational Safety and Health Administration, requires an employer to provide mandatory hearing protection, and run periodic hearing tests on employees whenever they are exposed to certain noise levels for a particular duration, as shown in Table 1. The 97-decibel output of a balloon burner would fall under OSHA regulations for employee exposures of three or more hours a day. Since balloonists seldom operate a balloon burner for more than 5 minutes a flight, they fall outside the OSHA regulations. However, this does not necessarily mean that there is no risk. "A certain percentage of the population, roughly five or six percent, can begin to show evidence of noise-induced hearing loss with exposures lower than the OSHA guidelines," warns audiologist Glenn Spatola, of the National Hearing Conservation Association. "I would certainly recommend that a person wear hearing protection for any exposure over 85 decibels, even for a few seconds."
Ballooning manufacturers are aware of the negative consequences of loud burners, and are concentrating on better burner designs to reduce their noise level. "What we've done is to spread out the flame," explains Mark West of Aerostar. "We let the flame take place over a much larger area so that the rate of expansion of gas is slower." In 1985, Aerostar came out with a new jet-type nozzle that has 19 small jets. "This was a major radical reduction in noise and basically it was the result of using more jets," remarks West.
While waiting for manufacturers to produce a quieter burner, there are steps a balloonist can take to protect passengers and crew from uncomfortable noise levels. First, hearing specialists recommend earplugs or earmuffs for anyone who works in a loud environment. Foam earplugs when properly fitted can provide 15-30 decibels of noise protection. Snug fitting earmuffs typically offer 15 decibels of protection. Also, for those who already have a hearing deficit, there are special hearing aids for sportsmen that are designed to compress extreme noises, like gunfire, to a safe, audible level. Finally, there are new active noise reduction (ANR) technologies that generate mirror image sound waves of opposite phase to cancel out external noise.
Active noise reduction is an electronic solution to the problem of a high-noise environment. A microphone reads surrounding low-frequency noises, and then a chip generates an anti-wave signal to cancel the noises. Think of ripples in water. If the troughs of duplicate ripples hit the peaks of the original ripples head-on, then the water goes flat. That's how an anti-noise wave cancels out surrounding sounds.
There are a number of commercially available active noise reduction devices. Noise Cancellation Technologies (NCT) manufactures a unit called the Noise Buster Extreme which retails for less than $70. NCT claims that its Noise Buster will cancel noise in the frequency range of 20-1500 hertz by 8-15 decibels. The Noise Buster unit consists of a headset with microphones and lightweight earphones connected to a portable radio-sized device containing cancellation circuitry. NCT also produces a more robust unit for industrial use, called the Pro-Active, which boasts active noise reduction up to 20 decibels. Another "high-end" active noise reduction headset is produced by KOSS, one of the leading U.S. manufacturers of high-fidelity stereo headphones. KOSS's Quiet Zone 2000 claims to reduce noise up to 20 decibels, but it also sports a hefty sticker price of $200.
But before running out and buying one of these active noise reduction units for your next balloon flight, balloonists who have tried these devices say they have been disappointed. Active noise reduction devices have currently been designed to reduce constant, predictable, steady state noise while allowing speech to come through unchanged. Unfortunately, this design makes active noise reduction less than desirable for the random broadband noise produced by a hot air balloon burner. Engineers in the active noise reduction industry claim that eventually their units will be designed to be more versatile and will be capable of being programmed to filter out more random noises, like the kind of noise generated by a balloon burner. However, at present, this technology is not commercially available.
What about the future of active noise reduction in aviation? Particular models of SAAB turboprop airplanes already use active noise reduction to lower propeller noise within the planes' cabins. According to SAAB, sound levels in cabins that use active noise reduction have been reduced by 18 decibels. Can this technology be adapted so pilots and passengers will hear little or no sound from a hot-air balloon burner without using earplugs or some other forms of worn protection? Greenhlagh Technologies in Neshanic Station, New Jersey, thinks so. Back in 1996, they were issued a patent on a system for suppressing the sound from a flame. The patent (U.S. Patent No. 5,488,666) uses a photo sensor, as shown in Figure 1, to detect the noise output of a flame. The signal is then processed using active noise reduction. Anti-noise waves are produced using an electric arc formed at a cathode positioned in the path of the flame produced by the burner.
Some are skeptical of Greehlagh's patented method of noise reduction, but not Anders Andersson, one of The Boeing Company's top noise engineers and holder of at least ten patents on noise reduction. "There is quite a bit of correlation between light output and sound output because burner noise is proportional to the rate of combustion, and light output is also proportional to that. So, it's not impossible and I think it would work." Anderson also suggests that the noise source doesn't have to come from a spark or a speaker system. "Your anti-noise source may come from a pneumatic noise source. In other words, you modulate air that comes out of some opening." He says, "if you can do that right in the flame, you might get up to those higher frequencies and cancel them out."
Even if the patented technology were designed and implemented, it would probably only create a zone of silence around the pilot and passengers and would not eliminate the sound heard by spectators and livestock on the ground. This is because, to eliminate ground noise, the power required to cancel out the burner noise would have to be equivalent to the power of the noise output of the burner, and the weight penalty for such a system would make it undesirable.
Although it is possible to reduce the perceived noise of a balloon burner, to some balloon pilots it isn't desirable. "Some balloon pilots are objecting to noise protection" according to West, "they're telling us that they want to hear their burner because they want to know what's going on with it. They want to hear when that change in sound happens to the burner, and they want to hear the pilot light." The ballooning industry expects that there will be resistance to any effort to have balloon pilots begin wearing noise protection, even if it becomes apparent that there is the potential danger of hearing loss.
So what's in the future of burner noise reduction? West of Aerostar is cautious. "I don't believe you are ever going to get rid of all the noise." He says, "I don't think that ballooning would bear the development expense, firstly, and secondly, I don't think you would have the people out there who would be willing to put cash on the line for it." West believes the noise problem probably won't be solved electronically. "I can put my money to work the best in terms of doing the best for the industry by continuing to work on the burner."
There is little chance, at least in the near future, that hot-air balloons will be floating over peaceful valleys in total silence. However, ballooning manufacturers are working on making their burners quieter and more efficient. Electronic devices may soon be available that will offer some protection to balloonists and passengers. Until then, to reduce the risk of hearing loss and other noise-induced health risks, the best solution appears to be the use of protection worn in the ears like foam earplugs.